<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    /* mobile   */
    @media (max-width:768px){
        div{
            background: skyblue;
        }
    }
    /* pad   */
    @media (min-width:769px) and (max-width:992px){
        div{
            background: salmon;
        }
    }
    /* pc  */
    @media (min-width:993px){
        div{
            background: darkgoldenrod;
        }
    }
    div{
        width: 100px;height: 100px;
    }
    </style>
</head>
<body>
    <div></div>
</body>
</html>